1. Field of the Invention
The invention relates to a frequency balanced six-port reflectometer. More specifically, the invention relates to such a six-port reflectometer having a variable impedance at the test port input thereof.
2. Description of Prior Art
The characterization of active microwave devices is generally accomplished by the measurement of S parameters using, for example, Automatic Network Analyzers (ANA). The test port impedance of the ANA seen by the active device under test is the normal line characteristic impedance of the measurement system (50 .OMEGA.). It is noted that such test port impedance cannot normally be varied during measurements as is explained in reference manuals of such ANA's and well known in the art. Such a constraint is not convenient when measuring active devices with negative resistance, e.g. diodes, transistors, etc., because oscillations might occur during such measurement whenever the negative resistance of the device exceeds 50 .OMEGA..
The use of six-port devices as an alternative to ANA's to measure such characteristics as reflection coefficients, S parameters, and impedance is designed in most cases with a fixed internal test port impedance generally near the characteristic line impedance (50 .OMEGA.) of the six-port measurement system. See, for example, U.S. Pat. No. 4,571,545, Griffin et al, U.S. Pat. No. 4,104,583, Engen and U.S. Pat. No. 4,521,728, Li. Such a six-port device is also described in G. F. Engen, "The Six-Port Reflectometer: An Alternative Network Analyzer," IEEE Trans. Microwave Theory Tech., Vol. MTT-25, No. 12, pps 1075-1083, Dec. 1977.